The present invention relates to a device for detecting optical position for detecting a direction of an optical axis in a system having an optical system such as an automatic survey instrument for automatically tracking a target.
A surveying system comprises an automatic survey instrument positioned at a known point and a collimation target disposed on a target side and provided with a reflection prism. The automatic survey instrument is used for automatically collimating or tracking the collimation target at the target point. For the purpose of tracking, the automatic survey instrument is provided with a device for detecting optical position for detecting deviation of the collimation target and a direction of collimation.
FIG. 5 shows an essential portion of an automatic survey instrument. The automatic survey instrument comprises a leveling unit 1 mounted on a tripod (not shown), a base unit 2 mounted on the leveling unit 1, a base stand 3 rotatably mounted around a vertical axis of the base unit 2, and a telescope unit 4 rotatably mounted around a horizontal axis of the base stand 3.
The telescope unit 4 projects a measuring light beam including light components for tracking and light components for distance measurement with different wavelength ranges and comprises an angle detector for measuring a collimation direction of the telescope, a distance-measuring unit for measuring distance to a reflection prism, and a tracking unit for detecting a collimation target and for performing collimation.
The tracking unit is used for projecting an invisible light beam and a visible light beam in the collimation direction and comprises a light emitting unit, a photodetection unit, a control unit, and an optical system for projecting and receiving the tracking light beam, a vertical rotating motor for rotating the telescope unit 4 around the horizontal axis, and a horizontal rotating motor for rotating the base stand 3 together with the telescope unit 4 around the vertical axis.
In the automatic survey instrument for automatically tracking a target as described above, the projected measuring light beam contains light components in different wavelength ranges for tracking and for distance measurement. A reflection light beam reflected by the target and received is divided to light components with different wavelengths for tracking, for distance measurement and for visual purpose. Then, distance measurement and automatic tracking are performed using the distance-measuring light beam and the tracking light beam thus divided.
The tracking light beam for the purpose of tracking is projected toward the reflection prism, which is a target for tracking. After being reflected by the reflection prism, the tracking light enters again from the telescope and forms an image on the photodetection unit. The center of the photodetection unit is aligned with a collimation axis of the telescope. When the image of the reflection prism comes to the center of the photodetection unit, it is identified as the center of collimation. Accurate survey operation must be performed at the center of collimation.
An area sensor may be used as a photodetection unit for detecting the center of collimation. The area sensor itself is expensive in cost and also it requires an expensive arithmetic operation unit because the distance of the photodetecting position from the center of collimation is obtained by calculation. In this respect, a quadrisected element is used, which is simple and sufficient for the purpose.
Referring to FIG. 6, brief description will be given on a quadrisected element photodetector 5.
In the quadrisected or 4-division element photodetector 5, a photodetection area is divided into 4 parts, and each part of photodetection elements 5a, 5b, 5c and 5d is arranged on one of the divided photodetection areas and the four photodetection elements each operate independently. When the quadrisected element photodetector 5 receives the tracking light beam reflected from the reflection prism, photodetection outputs from the parts of photodetection element 5a, 5b, 5c and 5d are compared with each other. If there is output difference between the photodetection elements, it is judged that the reflection prism is not aligned with collimation direction. When there is no output difference between the photodetection elements, it is judged that the reflection prism is aligned with the collimation direction. The quadrisected element photodetector may be designed in such manner that a quadrisected element photodetector is provided at the central portion of the photodetector to have higher accuracy.
The equations (1a) and (1b) as given below each represents an equation to obtain photodetecting position based on the output from the quadrisected element photodetector 5. The photodetecting position is aligned with the center of the quadrisected element photodetector 5 at a position where the values of the two equations is turned to 0.
Position in lateral direction=[(A+C)xe2x88x92(B+D)]/(A+B+C+D)xe2x80x83xe2x80x83(1a)
Position in longitudinal direction=[(A+B)xe2x88x92(C+D)]/(A+B+C+D)xe2x80x83xe2x80x83(1b)
Conventionally, when position of a ship for special task or operation or the like is to be detected by automatic tracking, there is no substantial problem even when the accuracy may be relatively low. However, in one-man survey operation using automatic tracking function, it is normal survey operation and requires high accuracy.
For the tracking light beam requiring high accuracy, a laser beam having a high directivity is used in most cases. From the viewpoint of the driving power, a semiconductor laser element is generally used.
The laser beam is coherent light which has the high directivity and is easy to interfere. As shown in FIG. 7, a tracking light beam 7, which is emitted from a semiconductor laser element 6, is turned to a parallel beam by an optical system 8 and this beam is projected. Diffraction occurs in the tracking light beam 7 when it passes through the optical system 8. The semiconductor laser element 6 has a light emitting unit with a cross-section of oblong elliptical shape. In this respect, on the cross-section of the projected tracking light beam 7, ring-like interference fringes appear due to diffraction as shown in FIG. 8. Further, the fringes of ring-like shape are widened in the lateral direction.
The quadrisected element photodetector 5 is the element for detecting the deviation of direction by comparing the output of each element of the four independent photodetection elements. When the photodetection outputs are equal to each other, e.g. when the values of the equations (1a) and (1b) are 0, it is identified as the center. In case the tracking light beam 7 is an image such that a light quantity distribution of the image is continuous normal distribution, the deviation from the central position of the photodetection element can be easily detected by the above equations (1a) and (1b). However, in case of an image having the ring-like interference fringes, the center may not be detected in some cases from the output difference of the photodetection elements 5a, 5b, 5c and 5d of the quadrisected element photodetector 5.
FIG. 9 is a diagram showing relationship between a photodetecting position and a photodetection signal on the quadrisected element photodetector 5 relating to the vertical or horizontal direction. The value of the equations (1a) or (1b) obtained from the level of the photodetection signal is given on the axis of ordinate, and deflection angle of collimation axis with respect to the center of prism is represented on the axis of abscissa. A solid line shows the case where the cross-section of luminous flux of the laser beam has laser intensity of normal distribution, and a broken line represents the case where diffraction has occurred on the laser beam.
In FIG. 9, in case the laser beam has normal distribution, the deflection angle of the collimation axis is turned to 0 when the photodetection signal level is 0. In case of the laser beam with diffraction, even when the photodetection level is 0, the deflection angle of the collimation axis is not turned to 0, and it is difficult to detect the position accurately.
FIG. 10 shows a condition where the tracking light beam having the ring-like interference fringes is received on the quadrisected element photodetector 5. On the photodetection elements 5a and 5c, a ring-like fringe 7b around the luminous flux of the tracking light beam 7 is received as it is equally divided to upper and lower portions. On the photodetection elements 5b and 5d, the central spot 7a is received as it is equally divided to upper and lower portions. When it is assumed that photodetection quantity is the same for the ring-like fringe 7b and the central spot 7a under this photodetecting condition, the result of calculation by the equations (1a) and (1b) is turned to 0, and it is judged that the light beam is received at the center of the quadrisected element photodetector 5. In this case, there is a problem in that the output from the quadrisected element photodetector 5 may be erroneously regarded as the center.
Such problem also occurs in case of the area sensor. Thus, the decrease of measurement accuracy is unavoidable in case diffraction occurs in the laser beam.
It is an object of the present invention to provide a device for detecting optical position, by which it is possible to perform position detection with high accuracy even when diffraction occurs in the detection light beam used for position detection and an quantity of the detection light is dappled.
To attain the above object, the device for detecting optical position according to the present invention comprises a photodetection unit having a photodetection optical axis and for outputting a photodetection signal corresponding to a light receiving position of a projected light beam and a modulation grid arranged on the photodetection optical axis and for equalizing distribution of light quantity of a received light beam, wherein a position of the light beam projected on the photodetection unit is detected based on the photodetection signal. Also, the present invention provides a device for detecting optical position as described above, wherein the photodetection unit comprises a plurality of photodetection elements, and the photodetecting position is detected based on deviation of the photodetection signal from each of the photodetection elements. Further, the present invention provides a device for detecting optical position as described above, wherein the modulation grid has a pattern of grid shape randomly arranged. Also, the present invention provides a device for detecting optical position as described above, wherein a photodetection unit and a modulation grid together with a projection unit are provided on a tracking optical system of an automatic survey instrument for determining collimation. Further, the present invention provides an automatic survey instrument for automatically tracking a target, comprising a distance-measuring unit and a tracking unit, wherein the tracking unit comprises an optical system for receiving a reflection light beam from the target, and the optical system comprises a photodetection optical axis, a photodetection unit for outputting a photodetection signal corresponding to a position of a projected light beam, and a modulation grid arranged on the photodetection optical axis, and the position of light beam projected on the photodetection unit is detected based on the photodetection signal. The device for detecting optical position of the present invention comprises a modulation grid, and this makes it possible to equalize light quantity distribution of light beam received and to prevent decrease of accuracy in position detection due to light quantity dapple.